1. Field of the Invention
The present invention relates to a disk device that reproduces information recorded on a disk, and more particularly, to a disk device that prevents a wrong reproducing operation when a defect is caused on a disk, and also relates to a disk reproduction method.
2. Description of the Related Art
In recent years, recording and reproduction apparatuses for optical disks such as DVDs (Digital Versatile Disks) have been widely used, and various types of apparatuses have been developed and put into practical use.
Particularly, high-density recording has been rapidly developed in the field of optical recording and reproduction, and linear recording density has greatly increased. Also, as the laser wavelengths are becoming shorter and the lens aperture is becoming greater, deterioration of reproduction signal quality due to tilting is becoming more and more conspicuous. To counter this problem, the PRML (Partial Response and Maximum Likelihood) signal processing method is being applied to many cases. This method is suitable for optical disk devices. By the PRML signal processing method, higher signal quality can be achieved even with high-density information recorded on an optical disk, compared with signal quality that can be obtained by a conventional level slice method. Therefore, there is a great possibility that the PRML signal processing method will be applied to next-generation optical disks that are designed for HD image recording.
On the other hand, since the objects to be handled with an optical disk device are removable disks, unlike with a hard disk device, stable signal reproduction is expected even from a disk having dust, dirt, or fingerprints on it, or having defects such as scratches. If a defect occurs, disturbed signals cannot be properly reproduced, and the adverse influence remains in the filter that manages the control values for reproduction signals. Therefore, even after a recovery from the defect, abnormal data might be generated from normal data over a certain period of time. Taking measures against defects is essential for an optical disk device that performs high-density recording and reproduction according to the PRML method.
Patent document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2003-157528) discloses a method of detecting a defect by monitoring the amplitude level of each RF signal. In this method, the envelope detection result of each RF signal is used to detect a variation of the amplitude of each RF signal, because the amount of reflected laser light is varied when a defect occurs. As a measure to be taken against defects, the control value of each signal processing section obtained immediately before the defect detection is held so as to restrict the influence of the defect upon the reproducing operation.
Patent document 2 (Jpn. Pat. Appln. KOKOKU Publication No. 1-50992) also discloses such a method.
According to the prior art disclosed in the above-described patent document, when a defect is detected from a change in the envelope of a RF signal, the control value of each signal processing section obtained immediately before the defect detection is held so as to restrict the adverse influence of the defect upon the reproducing operation. By this method, however, problems such as a detection delay in the defect detector are not taken into consideration. More specifically, since a delay is caused before a defect is detected, the control value immediately before the defect detection might be a control value generated during the period of the defect occurrence, and therefore, might be inappropriate as a held value of a control value for achieving high-quality signal reproduction during the defect recovery time and immediately after a recovery from the defect. For example, if a held control value obtained from the inner periphery of the disk represents a safe amount, the control value is not appropriate for being used on the outer periphery of the disk, because the information is already old and an in-plane variation could be caused on the disk.